An overview of your trip
It's often difficult for first-time visitors to decide which reserves to visit – so an overview here might help.
If you're planning on a beach-and-safari trip then first be mindful that safari time will usually cost much more than beach time; it's often double the nightly cost. For example, a half-decent safari lodge will easily cost US$300 each per night, whilst you'll have to try hard to find equally comfortable beach lodges at more than US$150 each per night. (The reason for this is partly that most safari lodges include a full day of activities and all your meals, whereas a beach lodge often provides only a room and breakfast; and partly because the logistics at most safari lodges are that much more expensive.)
With this in mind, consider how long you want to spend on each part of your trip. A 50/50 split (a week on safari and a week by the beach) is typical for a two-week trip, but this depends on your priorities and budget.
If you decide on five nights or less on safari, then our advice is probably to stick to Selous – and spend all your time there. If you decide on eight nights or more on safari, then our advice is probably to combine time in Selous with time in Ruaha – as then the extra cost of flying between the parks is worth it for the change in scenery and environment. If you decide on six or seven nights on safari, then it's less clear if Ruaha is worth the extra travelling or not; you need to think about it.
Finally, if you're really looking for just two or three days of safari, perhaps beside a beach where you can lie back and forget safari activities completely if you wish, then consider Saadani. This small, coastal park allows you the flexibility to do just that.